Photochromism is a reversible action of a compound which quickly changes its color when it is irradiated with light containing ultraviolet rays such as sunlight or light of a mercury lamp and resumes its initial color when it is no longer irradiated with light and is placed in a dark place, and has been applied for a variety of uses.
For example, photochromism has been applied in a field of spectacle lenses, too, and plastic lenses having photochromic properties are obtained by curing polymerizable monomers to which have been added various photochromic compounds having the above-mentioned properties. As photochromic compounds that can be favorably used for such applications, there have been used fulgimide compounds, spirooxazine compounds and chromene compounds.
To produce plastic lenses having photochromic properties, there have been proposed:    {circle around (1)} a method (imbibition method) of imbibing the surface of a lens (without photochromic property) with a photochromic compound;    {circle around (2)} a method (in mass method) of directly obtaining a photochromic lens by dissolving a photochromic compound in a monomer which is, then, polymerized; and    {circle around (3)} a method (coating method) of forming a layer having photochromic properties on the surfaces of a lens.
To obtain favorable photochromic properties by the above imbibition method {circle around (1)} or the in mass method {circle around (2)}, however, the substrates of lenses must be so designed that favorable photochromic properties are exhibited, and limitation is imposed on the substrates for lenses that can be used. For example, it is contrived to lower the glass transition temperature (Tg) of the substrate of lenses based on such a design policy that the molecules of the photochromic compound are permitted to easily move even in high molecules, or free spaces in the high molecules are expanded so that the molecules of the photochromic compounds are allowed to easily move.
As a monomer for forming a substrate of lenses, for example, U.S. Pat. No. 5,739,243 teaches use of a particular long-chain alkylene glycol dimethacrylate in combination with a polyfunctional methacrylate having three or more radically polymerizable groups. This method makes it possible to obtain a photochromic lens having relatively excellent color density and fading rate. According to this method, however, the glass transition temperature Tg of the substrate is lowered to improve photochromic properties and to improve properties of the substrate for being imbibed with a photochromic compound. As a result, the substrate becomes too soft newly arousing such problems as a decrease in the hardness of the substrate, a decrease in the heat resistance and large optical strain. The above problem can be solved by contriving a monomer and a photochromic material for obtaining a substrate of plastic lenses (see, for example, PCT International Patent Publication 01/05854). So far as the above method {circle around (1)} or {circle around (2)} is employed, however, some limitations are inevitably imposed on the substrate.
According to the above coating method {circle around (3)}, on the other hand, it is allowed to impart photochromic properties to the substrate of lenses that has been generally used without any limitation. When the coating method is employed, however, a technology has not yet been established for forming a coating film having a small and homogeneous thickness that will not adversely affect the properties of the substrate, having a sufficiently high surface hardness and, further, having good photochromic properties.
For example, WO98/37115 is proposing a method of coating the surfaces of a lens with a coating solution obtained by dissolving a photochromic compound in an urethane oligomer, followed by curing. However, a resin obtained by curing the urethane oligomer has a low crosslinking density causing the photochromic property to vary to a large extent depending upon the temperature and, further, involving such a defect that a photochromic compound elutes into the solution of a hard coating material when the hard coating material is being applied onto the photochromic coating layer.
U.S. Pat. No. 5,914,174 proposes a method of obtaining a photochromic lens having a high molecular weight photochromic film laminated on the convex surfaces of a general lens by dissolving a photochromic compound in a polymerizable composition containing monofunctional, bifunctional and polyfunctional radically polymerizable monomers, flowing the polymerizable composition into a cavity between a plastic lens and a glass mold held by an elastomer gasket or a spacer, and polymerizing the polymerizable composition. According to this method, however, the thickness of the obtained photochromic high molecular weight film (coating) becomes as large as 200 to 500 μm, and the strength of the high molecular weight film is reflected on the plastic lens.
That is, the strength of the photochromic lens is low as compared to the strengths of general lenses without having a high molecular weight film. According to this method, further, it is difficult to maintain small and constant the cavity between the plastic lens and the glass mold and is, hence, difficult to form a high molecular weight film having a small and uniform thickness. This tendency becomes conspicuous particularly when the lens surfaces have a complex shape.
Further WO01/02449 proposes a method of obtaining a photochromic lens having a photochromic high molecular weight film of a thickness of about 20 μm formed on the convex surfaces of a general lens by dissolving 5 to 10 parts by weight of a photochromic compound in a polymerizable composition of a combination of two or more kinds of bifunctional (meth)acrylic monomers, applying the polymerizable composition onto the convex surfaces of the lens by spin-coating, and photopolymerizing the lens that is applied by the substitution in a nitrogen atmosphere. According to this method, the surfaces of the lens are coated with a photochromic film that develops color sufficiently densely maintaining a thickness (about 20 μm) that will not adversely affect the properties of the substrate. However, the above publication is considering none of the uniformity of thickness or homogeneity of the coating film, or the optical properties of the obtained lenses.
Generally, a photopolymerization initiator and a photochromic compound are both excited with ultraviolet rays. When light is irradiated under a condition where both of them exist together, therefore, the photopolymerization initiator is minimally decomposed and the polymerization does not easily take place. By using a polymerizable and curable composition containing a photochromic compound and a photopolymerization initiator, therefore, the present inventors have studied the effect of the film-forming conditions upon the coating film. As a result, there were found the following problems involved in the coating method.    (i) When an extended period of time is spent by the polymerization, the surface of the substrate on where the film is to be formed loses flatness. When the lens has curved surfaces like spectacle lenses, the coating agent drips making it difficult to obtain a film having a uniform thickness.    (ii) When the ultraviolet ray is continuously irradiated for extended periods of time under a condition where the polymerization is not taking place to a sufficient degree, the radical reaction is interrupted being affected by oxygen contained in very small amounts in the atmosphere despite the atmosphere is substituted with nitrogen. In particular, an unpolymerized layer is formed near the surface and a film having a sufficiently large surface hardness is not obtained.    (iii) The irradiation with ultraviolet rays for extended periods of time in a state where the polymerization is not taking place to a sufficient degree causes the photochromic compound to be deteriorated due to photo oxidation.    (iv) When irradiated with active energy rays containing intense ultraviolet rays to shorten the time of light irradiation while promoting the polymerization, the surface of the lens is heated at high temperatures at the time of curing (e.g., 120° C. or higher, or 200° C. or higher) being affected by the heat from the source of light and by infrared rays. When a substrate of plastic lenses having low heat resistance is used, therefore, the lens itself is thermally deformed.    (v) A difference occurs in the contraction due to a difference in the polymerizing rate between the surface of the coating and the interior thereof, making it difficult to obtain a homogeneous high molecular weight film.